1. Field of the Invention
The present invention relates to a mobile communication system, a radio base station apparatus, an operation control method therefor, and a program, and particularly to an improvement of a mobile communication system that a mobile station transmits an RACH (random access channel) preamble to a base station prior to transmitting an RACH message to the base station, and in response to detection of the RACH preamble, the base station transmits an AICH (acquisition indicator channel) to the mobile station.
2. Description of the Related Art
Recently, the interference-resistant spread spectrum communication method has attracted a lot of attention as one of communication methods applicable to mobile communication systems. In a radio communication system using the spread spectrum communication method, after digitized voce data or image data is modulated with a digital modulation method such as the PSK and FSK modulation methods in a transmitting-side apparatus, for example, the modulated data is converted into a broadband baseband signal using a spread code such as a pseudorandom noise code (PN code), then converted into radio frequency signal, and transmitted.
Meanwhile, in a receiving-side apparatus, the received radio frequency signal is de-spread with the same code as the spread code used by the transmitting-side apparatus, and after that, it is digital-demodulated with the PSK or FSK demodulation method to reproduce the received data.
In a random access control method in a conventional CDMA mobile communication system, wherein each of multiple mobile stations accesses to a base station using an RACH (random access channel) at an arbitrary timing as required and the base station controls its message sending in response to the access, when a mobile station makes a call, it transmits an RACH preamble notifying that an RACH message has occurred, to the base station prior to transmitting the RACH message. In this case, the mobile station selects one signature from among sixteen kinds of signatures at random and generates the preamble from the selected signature.
On receiving the preamble, the base station compares a predetermined threshold with the correlation value. If the correlation value is larger, then it is determined that the preamble has been detected, and an AICH (acquisition indicator channel) corresponding to the signature of the detected preamble is transmitted to the mobile station. The base station searches for all the sixteen kinds of signatures, and the AICH is not transmitted if no preamble is detected. The AICH includes the signature number of the detected preamble, and information “ACK (acknowledge)” indicating that message sending by the mobile station is permitted by the base station or information “NACK (negative acknowledge)” indicating that message sending is not permitted.
If the mobile station receives an AICH corresponding to the signature of the preamble within a predetermined period of time after transmitting the preamble, it transmits a message in the case of “ACK”, and exits the random access procedure in the case of “NACK”. If the mobile station cannot receive an AICH corresponding to the signature number of the preamble in the predetermined period of time, it increases transmit power by the step width of power ramping (Power Ramp Step) and transmits the preamble again. Generally, mobile stations retransmit a preamble at predetermined intervals, and the step width of power ramping is constant.
Detail description will be now made on the random access control method described above, referring to drawings. FIG. 7 shows the configuration of a conventional radio base station apparatus, and FIG. 8 shows the configuration of a conventional mobile station (radio terminal apparatus). Referring to FIG. 7, the conventional radio base station apparatus comprises a delay profile generation circuit 31, an RACH preamble detection circuit 32 and an AICH generation circuit 35.
The delay profile generation circuit 31 performs correlation calculation between the received signal from the mobile station shown in FIG. 8 inputted into the delay profile generation circuit 31 and each of known RACH preamble codes corresponds to signatures, and generates delay profiles based on the correlation calculation results. In this case, the delay profile generation circuit 31 is activated in a constant cycle of arrival of an RACH preamble.
Each of the delay profiles created by the delay profile generation circuit 31 is outputted to the RACH preamble detection circuit 32. If a correlation peak equal to or above a predetermined threshold is detected from the delay profile, the RACH preamble detection circuit 32 determines that an RACH preamble has been detected. The RACH preamble detection circuit 32 then provides “ACK” indicating permission for the use of resources or “NACK” indicating refusal of the use of resources for each of the signatures of the detected RACH preambles and outputs them to the AICH generation circuit 35 as AICH information. The AICH generation circuit 35 generates and transmits an AICH based on the AICH information.
As shown in FIG. 8, the conventional mobile station comprises a delay profile generation circuit 21, an AICH detection circuit 22, a transmit power control circuit 23 and an RACH preamble generation circuit 24.
The delay profile generation circuit 21 performs correlation calculation using the same spread code as the spread code used by the radio base station apparatus shown in FIG. 7 for the received signal from the radio base station apparatus inputted into the delay profile generation circuit 21, and generates a delay profile based on the correlation calculation result. The delay profile is outputted to the AICH detection circuit 22, and it is determined whether an RACH preamble which the mobile station sent has been detected by the radio base station apparatus and whether resources have been assigned.
In this case, if the AICH detection circuit 22 cannot receive an AICH corresponding to the signature of an RACH preamble within a predetermined period of time after transmitting the RACH preamble, it determines that the radio base station apparatus could not detect the RACH preamble, and therefore outputs an RACH preamble retransmitting control signal to the transmit power control circuit 23 and the RACH preamble generation circuit 24.
On receiving the RACH preamble retransmitting control signal, the RACH preamble generation circuit 24 outputs an RACH preamble signal to the transmit power control circuit 23 after an interval of a predetermined number of access slots, and the transmit power control circuit 23 transmits the RACH preamble signal with power increased by a predetermined value relative to the power used when previously transmitting the RACH preamble signal.
The random access control method described above is disclosed in 3 GPP TS 25.214 V3.9.0, December 2001, pp. 28 to 30, for example.
In such a random access control method, there may be a case as shown below. That is, for a mobile station located near the border of a cell of the radio base station apparatus, for example, it is difficult to receive an AICH. When the mobile station cannot receive an AICH, there is caused an AICH abnormal state (the state in which a mobile station keeps ramping in spite of having sent an RACH preamble with power exceeding a detection threshold set for the radio base station apparatus, while the radio base station apparatus has normally detected the RACH preamble and therefore performs the operation of detecting an RACH message; and as a result, an RACH message cannot be sent), while a mobile station located in the center of the cell, which is in good receiving conditions, can detect an AICH.
In such a case, if there are more mobile stations in the center of the cell than near the border, the phenomenon that the radio base station apparatus cannot receive an RACH message does not appear obviously, and therefore it is difficult for maintenance personnel to find it. The same goes for the case where the phenomenon is caused by a mistake in setting of a transmit power value for an AICH or a failure in the radio base station apparatus. To the user who cannot send an RACH message, however, such condition seems to be a state in which he cannot start communication or a call for a long time (no service state). which will significantly damage his reliability on the communication service.
As described above, there is a problem that even if a mobile station cannot detect an AICH because of bad AICH receiving conditions, recovery from the AICH abnormal state cannot be performed until RACH messages cannot be sent so frequently that maintenance personnel notice the AICH abnormal state and change the setting for the AICH transmit power.